Preparation of Monosilyl Acetals from Esters via ÍBu2AIH Reduction and Trapping with iV-(Trimethylsilyl)imidazole. Addition of Allyltrimethylsilane to Yield Homoallylic Alcohols or Ethers

Alkyl esters were reduced with 1BU₂AIH or a 1:1 mixture of 1BU₂AIH and 1Bu₃AI (iBu₂AlH-iBu₃Al or 1BU₅AI₂H), followed by trapping of the resulting tetrahedral intermediate with TMS-imidazole to produce monosilyl acetals. Reaction of the mixed acetals with allyltrimethylsilane in the presence of Lewis acids (Hosomi-Sakurai reaction) generated homoallylic alcohols or ethers selectively, depending on the substitution of the monosilyl acetal. TMS methoxy acetals (MeO—CH—OTMS) and TMS ethoxy acetals bearing additional complexing groups such as MeO- or Ph₂C=N- provided alcohols with 1.5:1-9:1 threoselectivity, while simple TMS ethoxy acetals provided only ethyl ethers as products. The monosilyl acetal configuration was easily epimerized or racemized, and the configuration of the Hosomi-Sakurai product was apparently independent of the initial monosilyl acetal reactant configuration.